Optical transmission systems are increasingly used in telecommunication networks. They provide a relatively low cost, wideband transmission medium which supports many communication paths over each optical fiber. Voice, data and video signals are transmitted through the fibers as optical digital signals to minimize noise and distortion over long distances.
Because of the increased use of lightwave technology in transmission systems, there is interest also in using optical technology in switching systems for interconnecting a network of optical transmission systems. One advantage in using optical switching is the potential for eliminating the electrical-to-optical and optical-to-electrical conversions which are now required between the optical transmission systems and the existing electrical switching systems.
Frequency and frame synchronization are very important factors in an optical switching network. Very high bit rate bit streams from a variety of transmission systems must be coordinated for coincidental switching through the optical switches. Each sequential stream of information bits is grouped into a framed format. Within each frame, the bits are grouped according to their destination. Such groups of bits are called data segments. Following each data segment is a gap referred to as a guard band, or switch reconfiguration time. The purpose of the guard band is to provide time for reconfiguring the switch while allowing for time misalignment of frames arriving from different transmission system inputs to the switch. This type of multiplexing is known as block multiplexing.
Having grouped the information bits within each frame into data segments according to destination, it is possible to route the bits to their destination by properly configuring closed crosspoints in the optical switch during the guard bands. Switch reconfiguration must occur while the guard bands concur for all data streams arriving at the optical switch. Otherwise the switch reconfiguration occurs while one or more data segments are arriving at the switch and some data will be lost.
An optical switch may be of a variety of types; however, an optical space switch, based on titanium-diffused lithium niobate (Ti:Li Nb O.sub.3) directional couplers, is a reasonable option for early optical switching systems. The directional couplers can be arranged in a crossbar format. In non-blocking optical switches, the state of each directional coupler is subject to change periodically several times per frame of information. The states of the directional couplers are changed so that every input can be connected to every output at least once during every frame.
There are problems associated with the design of a synchronization arrangement for an optical switch. First of all it is essential that the frames of data arriving at every optical switch are all synchronized with each other and with the switch itself. They must be synchronized in both frequency and in phase. Any synchronization arrangement can use no more than a reasonable amount of equipment at a reasonable cost. Secondly it is important to design the synchronization arrangement for minimal operating cost. Because there are guard bands between the data segments in the signal stream, there is a potential for inefficient operation since operating efficiency is defined as the quantity frame duration time less total overhead time in one frame divided by total frame duration time.
Typical designs for optical switching networks cover large geographic areas and include optical transmission systems that are several hundred to several thousand miles long. The interconnecting cables which include the optical transmission media are subjected to wide differences of environmental conditions and temperatures. As a result, the total duration of guard bands per frame tends to be very long in relation to the length of the frames. Since efficiency of operating the network is determined by dividing the quantity frame duration time less the total duration of the guard bands and other overhead time per frame by the frame duration time, long guard bands tend to make the network inefficient and relatively more expensive to operate. Such inefficiency presents a problem for designers of optical switching networks.